os/bluestore: _do_zero: handle unzeroed tail blocks

os/bluestore: _do_zero: handle unzeroed tail blocks

- _do_write_zero when handling tail extent if it is
  shared.
- handle tail extent we are also zeroing later (not in
  the tail handler).  THis mirrors how _do_write is
  structured.
- handle eof to offset zeroing if our offset starts in
  the tail extent.  just like _do_write's wal path.

Signed-off-by: Sage Weil <sage@redhat.com>