xfs: clean up xfs_trans_brelse()

xfs_trans_brelse() is a bit of a historical mess, similar to
xfs_buf_item_unlock(). It is unnecessarily verbose, has snippets of
commented out code, inconsistency with regard to stale items, etc.

Clean up xfs_trans_brelse() to use similar logic and flow as
xfs_buf_item_unlock() with regard to bli reference count handling.
This patch makes no functional changes, but facilitates further
refactoring of the common bli reference count handling code.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>