Linux: Initialize zp in zfs_setattr_dir

Linux: Initialize zp in zfs_setattr_dir

The value of zp is used without having been initialized under some
conditions.  Initialize the pointer to NULL.

Add a regression test case using chown in acl/posix.  However, this is
not enough because the setup sets xattr=sa, which means zfs_setattr_dir
will not be called.  Create a second group of acl tests in acl/posix-sa
duplicating the acl/posix tests with symlinks, and remove xattr=sa from
the original acl/posix tests.  This provides more coverage for the
default xattr=on code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10043
Closes #11025