fs: gfs2: Adding new return type vm_fault_t

Use new return type vm_fault_t for gfs2_page_mkwrite
handler.

see commit 1c8f4220 ("mm: change return type to
vm_fault_t") for reference.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>