xfs: fix folio dirtying for XFILE_ALLOC callers

willy pointed out that folio_mark_dirty is the correct function to use
to mark an xfile folio dirty because it calls out to the mapping's aops
to mark it dirty.  For tmpfs this likely doesn't matter much since it
currently uses nop_dirty_folio, but let's use the abstractions properly.

Reported-by:  <willy@infradead.org>
Fixes: 6907e3c0 ("xfs: add file_{get,put}_folio")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>