mm: define default value for FIRST_USER_ADDRESS

Currently most platforms define FIRST_USER_ADDRESS as 0UL duplication the
same code all over.  Instead just define a generic default value (i.e 0UL)
for FIRST_USER_ADDRESS and let the platforms override when required.  This
makes it much cleaner with reduced code.

The default FIRST_USER_ADDRESS here would be skipped in <linux/pgtable.h>
when the given platform overrides its value via <asm/pgtable.h>.

Link: https://lkml.kernel.org/r/1620615725-24623-1-git-send-email-anshuman.khandual@arm.com
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>	[m68k]
Acked-by: Guo Ren <guoren@kernel.org>			[csky]
Acked-by: Stafford Horne <shorne@gmail.com>		[openrisc]
Acked-by: Catalin Marinas <catalin.marinas@arm.com>	[arm64]
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>	[RISC-V]
Cc: Richard Henderson <rth@twiddle.net>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Guo Ren <guoren@kernel.org>
Cc: Brian Cain <bcain@codeaurora.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Ley Foon Tan <ley.foon.tan@intel.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/sparc/include/asm/pgtable_32.h

@@ -48,7 +48,6 @@ unsigned long __init bootmem_init(unsigned long *pages_avail);
 #define PTRS_PER_PMD    	64
 #define PTRS_PER_PGD    	256
 #define USER_PTRS_PER_PGD	PAGE_OFFSET / PGDIR_SIZE
-#define FIRST_USER_ADDRESS	0UL
 #define PTE_SIZE		(PTRS_PER_PTE*4)
 
 #define PAGE_NONE	SRMMU_PAGE_NONE

arch/sparc/include/asm/pgtable_64.h

@@ -95,9 +95,6 @@ bool kern_addr_valid(unsigned long addr);
 #define PTRS_PER_PUD	(1UL << PUD_BITS)
 #define PTRS_PER_PGD	(1UL << PGDIR_BITS)
 
-/* Kernel has a separate 44bit address space. */
-#define FIRST_USER_ADDRESS	0UL
-
 #define pmd_ERROR(e)							\
 	pr_err("%s:%d: bad pmd %p(%016lx) seen at (%pS)\n",		\
 	       __FILE__, __LINE__, &(e), pmd_val(e), __builtin_return_address(0))