thp: change insert_pfn's return type to void

thp: change insert_pfn's return type to void

Orabug: 22913653

It would make more sense to have all the return values from
vmf_insert_pfn_pmd() encoded in one place instead of having to follow
the convention into insert_pfn().  Suggested by Jeff Moyer.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit ae18d6dcf57b56b984ff27fd55b4e2caf5bfbd44)
Signed-off-by: Dan Duval <dan.duval@oracle.com>