pipe_direct_write: Fix mismatched pipelock/unlock

If a signal is caught in pipelock, causing it to fail, pipe_direct_write
should not try to pipeunlock.

Reported by: pho
Differential Revision: https://reviews.freebsd.org/D3069
Reviewed by: kib
Approved by: markj (mentor)
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division