ubifs: Massage assert in ubifs_xattr_set() wrt. fscrypto

When we're creating a new inode in UBIFS the inode is not
yet exposed and fscrypto calls ubifs_xattr_set() without
holding the inode mutex. This is okay but ubifs_xattr_set()
has to know about this.

Signed-off-by: Richard Weinberger <richard@nod.at>