mm/gup.c: use is_vm_hugetlb_page() to check whether to follow huge

No functional change, just leverage the helper function to improve
readability as others.

Link: http://lkml.kernel.org/r/20200113070322.26627-1-richardw.yang@linux.intel.com


Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>