mm/memory.c: replace vmf->vma with variable vma

mm/memory.c: replace vmf->vma with variable vma

The code has declared a vma_struct named vma which is assigned a value of
vmf->vma.  Thus, use variable vma directly here.

Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: http://lkml.kernel.org/r/20200818084607.37616-1-yanfei.xu@windriver.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>