mm/mlock.c: drop unneeded initialization in munlock_vma_pages_range()

Before usage page pointer initialized by NULL is reinitialized by
follow_page_mask().  Drop useless init of page pointer in the beginning
of loop.

Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>