xfs: prevent extsize alignment from exceeding maximum extent size

When doing delayed allocation, if the allocation size is for a
maximally sized extent, extent size alignment can push it over this
limit. This results in an assert failure in xfs_bmbt_set_allf() as
the extent length is too large to find in the extent record.

Fix this by ensuring that we allow for space that extent size
alignment requires (up to 2 * (extsize -1) blocks as we have to
handle both head and tail alignment) when limiting the maximum size
of the extent.

Signed-off-by: Dave Chinner <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Alex Elder <[e-mail]>

fs/xfs/xfs_bmap.c

···
 				/* Figure out the extent size, adjust alen */
 				extsz = xfs_get_extsz_hint(ip);
 				if (extsz) {
+					/*
+					 * make sure we don't exceed a single
+					 * extent length when we align the
+					 * extent by reducing length we are
+					 * going to allocate by the maximum
+					 * amount extent size aligment may
+					 * require.
+					 */
+					alen = XFS_FILBLKS_MIN(len,
+						   MAXEXTLEN - (2 * extsz - 1));
 					error = xfs_bmap_extsize_align(mp,
 							&got, &prev, extsz,
 							rt, eof,