xfs_filemap_pfn_mkwrite() duplicates a lot of __xfs_filemap_fault().
It will also need to handle flushing for synchronous page faults. So
just make that function use __xfs_filemap_fault().
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>