mds: catch damage to dentry's first field

mds: catch damage to dentry's first field

When possible. Abort the MDS before it can be written to the
journal/directory.

This is part of a series to address corruption first observed in [1].
How the corruption is introduced is yet unknown.

[1] https://tracker.ceph.com/issues/38452#note-10

Fixes: http://tracker.ceph.com/issues/58482
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>