mm/mprotect.c: avoid repeated assignment in do_mprotect_pkey()

After adjustment, the repeated assignment of "prev" is avoided, and the
readability of the code is improved.

Link: https://lkml.kernel.org/r/20211012152444.4127-1-fishland@aliyun.com


Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>