xfs: fix inode use-after-free during log recovery

In this second series, I try to fix a use-after-free that I discovered
during development of the dfops freezer, where BUI recovery releases the
inode even if it requeues itself.  If the inode gets reclaimed, the fs
corrupts memory and explodes.  The fix is to make the dfops capture
struct take over ownership of the inodes if there's any more work to be
done.  This is a bit clunky, but it's a simpler mechanism than saving
inode pointers and inode numbers and introducing tagged structures so
that we can distinguish one from the other.

v2: rebase atop the new defer capture code
v3: only capture one inode, move as much of the defer capture code to
xfs_defer.c as we can
v4: make defer capture ihold the inode, and the caller still gets to
iunlock and irele it