ocfs2: validate bg_free_bits_count after update

This patch adds a safe check to ensure bg_free_bits_count doesn't exceed
bg_bits in a group descriptor. This is to avoid on disk corruption that was
seen recently.

debugfs: group <52803072>
       Group Chain: 179   Parent Inode: 11  Generation: 2959379682
       CRC32: 00000000   ECC: 0000
       ##   Block#            Total    Used     Free     Contig   Size
       0    52803072          32256    4294965350   34202    18207    4032
       ......

Signed-off-by: Srinivas Eeda <srinivas.eeda@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>