logfs: Prevent memory corruption

This is a bad one.  I wonder whether we were so far protected by
no_free_segments(sb) usually being smaller than LOGFS_NO_AREAS.

Found by Dan Carpenter <dan.carpenter@oracle.com> using smatch.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>