ubifs: Rectify space budget for ubifs_xrename()

There is no space budget for ubifs_xrename(). It may let
make_reservation() return with -ENOSPC, which could turn
ubifs to read-only mode in do_writepage() process.
Fix it by adding space budget for ubifs_xrename().

Fetch a reproducer in [Link].

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216569


Fixes: 9ec64962 ("ubifs: Implement RENAME_EXCHANGE")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>