Linux: zfs_zaccess_trivial() should always call generic_permission()

Building with Clang on Linux generates a warning that err could be
uninitialized if mnt_ns is a NULL pointer. However, mnt_ns should never
be NULL, so there is no need to put this behind an if statement. Taking
it outside of the if statement means that the possibility of err being
uninitialized goes from being always zero in a way that the compiler
could not realize to a way that is always zero in a way that the
compiler can realize.

Sponsored-By: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Youzhong Yang <yyang@mathworks.com>
Signed-off-by: Richard Yao <richard.yao@klarasystems.com>
Closes #14738