mm/memory.c: simplify pfnmap_lockdep_assert

Use local `mapping' to reduce the pointer chasing.

akpm: extracted from a bugfix which Linus fixed with b1b46751 ("mm:
fix follow_pfnmap API lockdep assert").

Link: https://lkml.kernel.org/r/20241004-fix-null-deref-v4-1-d0a8ec01ac85@iiitd.ac.in


Signed-off-by: Manas <manas18244@iiitd.ac.in>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Anup Sharma <anupnewsmail@gmail.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>