missing unlock in jfs_quota_write()

We should unlock &inode->i_mutex on the error path.  This bug was
in ext2_quota_write().  I sent a patch to them today as well.

Found by smatch (http://repo.or.cz/w/smatch.git).  Compile tested.

regards,
dan carpenter

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>