udf: Fortify LVID loading

A user has reported an oops in udf_statfs() that was caused by
numOfPartitions entry in LVID structure being corrupted. Fix the problem
by verifying whether numOfPartitions makes sense at least to the extent
that LVID fits into a single block as it should.

Reported-by: Juergen Weigert <jw@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>