Add LK_TRYUPGRADE operation for lockmgr(9), which attempts to
atomically upgrade shared lock to exclusive.  On failure, error is
returned and lock is not dropped in the process.

Tested by:      pho (previous version)
No objections from:     attilio
Sponsored by:   The FreeBSD Foundation
MFC after:      1 week
Approved by:	re (glebius)
Reimplement r255797 using LK_TRYUPGRADE.

The  r255797 was:
Increase the chance of the buffer write from the bufdaemon helper
context to succeed.  If the locked vnode which owns the buffer to be
written is shared locked, try the non-blocking upgrade of the lock to
exclusive.

PR:	kern/178997
Reported and tested by:	Klaus Weber <fbsd-bugs-2013-1@unix-admin.de>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (glebius)
For vunref(), try to upgrade the vnode lock if the function was called
with the vnode shared-locked.  If upgrade succeeded, the inactivation
can be done immediately, instead of being postponed.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (glebius)