MFC r286345: Relax serialization of SYNCHRONIZE CACHE commands.

MFC r286345: Relax serialization of SYNCHRONIZE CACHE commands.

Before this change SYNCHRONIZE CACHE commands were executed exclusively,
as if they had ORDERED tag.  But looking through SCSI specs I've found
no any reason to be so strict.  For reads this ordering seems pointless.
For writes it looks less obvious, so I left ordering against preceeding
write commands, while following ones are no longer required to wait.

git-svn-id: svn://svn.freebsd.org/base/stable/10@286930 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f