xfs: inode fork allocation depends on XFS_IFEXTENT flag

Due to confusion on when the XFS_IFEXTENT needs to be set, the
changes in e6a688c3 ("xfs: initialise attr fork on inode
create") failed to set the flag when initialising the empty
attribute fork at inode creation. Set this flag the same way
xfs_bmap_add_attrfork() does after attry fork allocation.

Fixes: e6a688c3 ("xfs: initialise attr fork on inode create")
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Tested-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>