jfs: makes diUnmount/diMount in jfs_mount_rw atomic

jfs_mount_rw can call diUnmount and then diMount. These calls change the
imap pointer. Between these two calls there may be calls of function
jfs_lookup(). The jfs_lookup() function calls jfs_iget(), which, in turn
calls diRead(). The latter references the imap pointer. That may cause
diRead() to refer to a pointer freed in diUnmount().  This commit makes
the calls to diUnmount()/diMount() atomic so that nothing will read the
imap pointer until the whole remount is completed.

Signed-off-by: Oleg Kanatov <okanatov@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>