ceph: shut down mount on bad mdsmap or fsmap decode

As Greg pointed out, if we get a mangled mdsmap or fsmap, then something
has gone very wrong, and we should avoid doing any activity on the
filesystem.

When this occurs, shut down the mount the same way we would with a
forced umount by calling ceph_umount_begin when decoding fails on either
map. This causes most operations done against the filesystem to return
an error. Any dirty data or caps in the cache will be dropped as well.

The effect is not reversible, so the only remedy is to umount.

[ idryomov: print fsmap decoding error ]

URL: https://tracker.ceph.com/issues/52303


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Greg Farnum <gfarnum@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>