UFS After a power surge the root filesystem exceeds the capacity

[Kenzo68]

Last night, after a power surge, the system is rebooted but the root filesystem now exceeds the capacity:

Before:

Filesystem Size Used Avail Capacity Mounted on
/dev/mirror/root 29G 14G 12G 54% /

After:

Filesystem Size Used Avail Capacity Mounted on
/dev/mirror/root 29G 28G -2.1G 108% /

This is the fsck(8) output:

(7)enzo@Joshua:~% fsck /dev/mirror/root
** /dev/mirror/root (NO WRITE)
** Last Mounted on /
** Root file system
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
UNALLOCATED  I=1124132  OWNER=operator MODE=100400
SIZE=0 MTIME=Sep  7 17:00 2015
FILE=/var/db/entropy/saved-entropy.1

REMOVE? no

What is the problem? How do I fix it?
Thank you