Fix sa.c to build on FreeBSD again

[mcmilk]

Motivation and Context

OpenZFS master can not be build on FreeBSD currently.

Description

Fix multiple build errors on FreeBSD.

The main reason is, that the variable 'dxattr_obj' is used
uninitialized within the start of the 'out label'.

The goto out is used a lot before the variables get it's initialization.
But with right in the beginning of the out label there is an if (dxattr_obj) ....

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.