ubifs: Fix off-by-one error

An inode is allowed to have ubifs_xattr_max_cnt() xattrs, so we must
complain only when an inode has more xattrs, having exactly
ubifs_xattr_max_cnt() xattrs is fine.
With this the maximum number of xattrs can be created without hitting
the "has too many xattrs" warning when removing it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>