xfs: validate fsmap offsets specified in the query keys

Improve the validation of the fsmap offset fields in the query keys and
move the validation to the top of the function now that we have pushed
the low key adjustment code downwards.

Also fix some indenting issues that aren't worth a separate patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>