ubifs: Rectify space budget for ubifs_symlink() if symlink is encrypted

Fix bad space budget when symlink file is encrypted. Bad space budget
may let make_reservation() return with -ENOSPC, which could turn ubifs
to read-only mode in do_writepage() process.

Fetch a reproducer in [Link].

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216490


Fixes: ca7f85be8d6cf9 ("ubifs: Add support for encrypted symlinks")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>