Fix some nits in zfs_getpages()

[markjdb]

Fix a couple of bugs in zfs_getpages() pointed out in post-commit review.

Motivation and Context

This change fixes a couple of small issues with a previous recent change to zfs_getpages().

Description

See the commit log message. Note that the null pointer dereference described there will not arise in practice today, as zfs_getpages() is always called with count=1 on FreeBSD.

How Has This Been Tested?

Currently running the ZTS.

Types of changes

• [x] 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:

• [x] 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.
• [x] All commit messages are properly formatted and contain Signed-off-by.