fs/binfmt_elf.c: better codegen around current->mm

"current->mm" pointer is stable in general except few cases one of which
execve(2).  Compiler can't treat is as stable but it _is_ stable most of
the time.  During ELF loading process ->mm becomes stable right after
flush_old_exec().

Help compiler by caching current->mm, otherwise it continues to refetch
it.

	add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-141 (-141)
	Function                                     old     new   delta
	elf_core_dump                               5062    5039     -23
	load_elf_binary                             5426    5308    -118

Note: other cases are left as is because it is either pessimisation or
no change in binary size.

Link: http://lkml.kernel.org/r/20191215124755.GB21124@avx2


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>