xfs: remove XFS_IFEXTENTS

The in-memory XFS_IFEXTENTS is now only used to check if an inode with
extents still needs the extents to be read into memory before doing
operations that need the extent map.  Add a new xfs_need_iread_extents
helper that returns true for btree format forks that do not have any
entries in the in-memory extent btree, and use that instead of checking
the XFS_IFEXTENTS flag.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 708b773..bf490bf 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1126,7 +1126,7 @@ xfs_fill_fsxattr(
 	if (ip->i_diflags2 & XFS_DIFLAG2_COWEXTSIZE)
 		fa->fsx_cowextsize = XFS_FSB_TO_B(mp, ip->i_cowextsize);
 	fa->fsx_projid = ip->i_projid;
-	if (ifp && (ifp->if_flags & XFS_IFEXTENTS))
+	if (ifp && !xfs_need_iread_extents(ifp))
 		fa->fsx_nextents = xfs_iext_count(ifp);
 	else
 		fa->fsx_nextents = xfs_ifork_nextents(ifp);