ubifs: ubifs_jnl_write_inode: Only check once for the limitation of xattr count

No need to check the limitation of xattr count every time in function
ubifs_jnl_write_inode(), because the 'ui->xattr_cnt' won't be modified
by others in the inode evicting process.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>