xfs: don't deplete the reserve pool when trying to shrink the fs

xfs: don't deplete the reserve pool when trying to shrink the fs

Every now and then, xfs/168 fails with this logged in dmesg:

Reserve blocks depleted! Consider increasing reserve pool size.
EXPERIMENTAL online shrink feature in use. Use at your own risk!
Per-AG reservation for AG 1 failed.  Filesystem may run out of space.
Per-AG reservation for AG 1 failed.  Filesystem may run out of space.
Error -28 reserving per-AG metadata reserve pool.
Corruption of in-memory data (0x8) detected at xfs_ag_shrink_space+0x23c/0x3b0 [xfs] (fs/xfs/libxfs/xfs_ag.c:1007).  Shutting down filesystem.

It's silly to deplete the reserved blocks pool just to shrink the
filesystem, particularly since the fs goes down after that.

Fixes: fb2fc1720185 ("xfs: support shrinking unused space in the last AG")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>