Add dev_strategy_csw() function, which is similar to dev_strategy()
but assumes that a thread reference was already obtained on the passed
device.  Use the function from physio(), to avoid two extra dev_mtx
lock and unlock.  Note that physio() is always used as the cdevsw
method, or is called from a cdevsw method, and the caller already owns
the reference.

dev_strategy() is left to keep KPI intact, but now it is implemented
as a wrapper around dev_strategy_csw().

Do some style cleanup in physio().

Requested and reviewed by:	kan (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Fix a race with the vnode reclamation in the aio_qphysio().  Obtain
the thread reference on the vp->v_rdev and use the returned struct
cdev *dev instead of using vp->v_rdev.  Call dev_strategy_csw()
instead of dev_strategy(), since we now own the reference.

Since the csw was already calculated, test d_flags to avoid mapping
the buffer if the driver supports unmapped requests [*].

Suggested by:	kan [*]
Reviewed by:	kan (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks