mm/gup.c: make follow_page_mask() static

follow_page_mask() is only used in gup.c, make it static.

Link: http://lkml.kernel.org/r/20190510190831.GA4061@bharath12345-Inspiron-5559
Signed-off-by: Bharath Vedartham <linux.bhar@gmail.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/gup.c

@@ -515,7 +515,7 @@ static struct page *follow_p4d_mask(struct vm_area_struct *vma,
  * an error pointer if there is a mapping to something not represented
  * by a page descriptor (see also vm_normal_page()).
  */
-struct page *follow_page_mask(struct vm_area_struct *vma,
+static struct page *follow_page_mask(struct vm_area_struct *vma,
 			      unsigned long address, unsigned int flags,
 			      struct follow_page_context *ctx)
 {