mm/gup.c: make unnecessarily global vma_permits_fault() static

Make vma_permits_fault() static as it is only used in mm/gup.c

This fixes a sparse warning.

Link: http://lkml.kernel.org/r/20161017122353.31598-1-tklauser@distanz.ch


Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>