e2scrub: create online fsck tool of sorts

e2scrub: create online fsck tool of sorts

Implement online fsck for ext* filesystems which live on LVM-managed
logical volumes.  The basic strategy mirrors that of e2croncheck --
create a snapshot, fsck the snapshot, report whatever errors appear,
remove snapshot.  Unlike e2croncheck, this utility accepts any LVM
device path, knows about snapshots running out of space, and can call
fstrim having validated that the fs metadata is ok.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>