bpf: Add bpf_sys_close() helper.

Add bpf_sys_close() helper to be used by the syscall/loader program to close
intermediate FDs and other cleanup.
Note this helper must never be allowed inside fdget/fdput bracketing.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210514003623.28033-11-alexei.starovoitov@gmail.com

tools/include/uapi/linux/bpf.h

@@ -4754,6 +4754,12 @@ union bpf_attr {
  * 		Find BTF type with given name and kind in vmlinux BTF or in module's BTFs.
  * 	Return
  * 		Returns btf_id and btf_obj_fd in lower and upper 32 bits.
+ *
+ * long bpf_sys_close(u32 fd)
+ * 	Description
+ * 		Execute close syscall for given FD.
+ * 	Return
+ * 		A syscall result.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -4924,6 +4930,7 @@ union bpf_attr {
 	FN(snprintf),			\
 	FN(sys_bpf),			\
 	FN(btf_find_by_name_kind),	\
+	FN(sys_close),			\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper