In the syncer, drop the sync mutex while patting the watchdog.

Some watchdog drivers (like ipmi) need to sleep while patting the watchdog.
See sys/dev/ipmi/ipmi.c:ipmi_wd_event(), which calls malloc(M_WAITOK).

Submitted by:	asomers
MFC after:	1 month
Sponsored by:	Spectra Logic
MFSpectraBSD:	637548 on 2012/10/04
Instead of requiring an edit to turn on ZFS debugging, define ZFS_DEBUG.

MFC after:	1 month