FreeBSD: zfs_register_callbacks() must implement error check correctly

FreeBSD: zfs_register_callbacks() must implement error check correctly

I read the following article and noticed a couple of ZFS bugs mentioned:

https://pvs-studio.com/en/blog/posts/cpp/0377/

I decided to search for them in the modern OpenZFS codebase and then
found one that matched the description of the first one:

V593 Consider reviewing the expression of the 'A = B != C' kind. The
expression is calculated as following: 'A = (B != C)'. zfs_vfsops.c 498

The consequence of this is that the error value is replaced with `1`
when there is an error. When there is no error, 0 is correctly passed.
This is a very minor issue that is unlikely to cause any real problems.

The incorrect error code would either be returned to the mount command
on a failure or any of `zfs receive`, `zfs recv`, `zfs rollback` or `zfs
upgrade`.

The second one has already been fixed.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14261