UBIFS: xattr bugfixes

Xattr code has not been tested for a while and there were
serveral bugs. One of them is using wrong inode in
'ubifs_jnl_change_xattr()'. The other is a deadlock in
'ubifs_setxattr()': the i_mutex is locked in
'cap_inode_need_killpriv()' path, so deadlock happens when
'ubifs_setxattr()' tries to lock it again.

Thanks to Zoltan Sogor for finding these bugs.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>