ZFS: Fix a panic while unmounting a busy filesystem.

This particular scenario was easily reproduced using a NFS export.  When the
first 'zfs unmount' occurred, it returned EBUSY via this path, while
vflush() had flushed references on the filesystem's root vnode, which in
turn caused its v_interlock to be destroyed.  The next time 'zfs unmount'
was called, vflush() tried to obtain this lock, which caused this panic.

Since vflush() on FreeBSD is a definitive call, there is no need to check
vfsp->vfs_count after it completes.  Simply #ifdef sun this check.

Submitted by:	avg
Reviewed by:	avg
Approved by:	ken (mentor)
MFC after:	1 month
Be more explicit about what each bio_cmd & bio_flags value means.

Reviewed by:	ken (mentor)
Don't check and warn about pmap mismatch on every call to busdma sync.
With some recent busdma refactoring, sometimes it happens that a sync
op gets called when bus_dmamap_load() never got called, which results
in a spurious warning about a map mismatch when no sync operations will
actually happen anyway.  Now the check is done only if a sync operation
is actually performed, and the result of the check is a panic, not just
a printf.

Reviewed by:	cognet (who prevented me from donning a point hat)