Description of problem:
Having the /var/lib/dirsrv/slapd-*/bak is a separate FS and almost 100% full,
the backup task enters an infinitive loop retrying to back up all files.

How reproducible:
Sometimes. Customer experienced the loop continuously while I only see the
loop 2 or 3 times before a directory copy fails.

Steps to Reproduce:
1. Create a separate FS for /var/lib/dirsrv/slapd-*/bak which has only several
   MB of free space.
2. Start a back up task:
   # /usr/lib64/dirsrv/slapd-*/db2bak.pl -D "cn=Directory Manager" -w
'<password>'
3. Monitor the /var/log/dirsrv/slapd-*/errors

Expected results:
Back to finish or bail out due to disk space issues.

Additional info:
The loops continues until the free space on the bak FS is large enough to hold
the new backup.