MFC 196417:

MFC 196417:
This patch fixes two bugs in sglist(9) and improves robustness of the API via
better semantics if a request to append an address range to an existing list
fails.
- When cloning an sglist, properly set the length in the new sglist instead of
  leaving the new list empty.
- Properly compute the amount of data added to an sglist via
  _sglist_append_buf().  This allows sglist_consume_uio() to properly update
  uio_resid.
- When a request to append an address range to a scatter/gather list fails,
  restore the sglist to the state it had at the start of the function call
  instead of resetting it to an empty list.

Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/stable/8@196418 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f