MFC of 233627, 234024, 234025, and 234026

MFC of 233627, 234024, 234025, and 234026

Restore per mount-point counts of synchronous and asynchronous
reads and writes associated with UFS and MSDOS filesystems.

MFS 233627:
Keep track of the mount point associated with a special device
to enable the collection of counts of synchronous and asynchronous
reads and writes for its associated filesystem. The counts are
displayed using `mount -v'.

Ensure that buffers used for paging indicate the vnode from
which they are operating so that counts of paging I/O operations
from the filesystem are collected.

This checkin only adds the setting of the mount point for the
UFS/FFS filesystem, but it would be trivial to add the setting
and clearing of the mount point at filesystem mount/unmount
time for other filesystems too.

Reviewed by: kib

MFC 234024:
Drop an unnecessary setting of si_mountpt when updating a UFS mount point.
Clearly it must have been set when the mount was done.

Reviewed by: kib

MFC 234025:
Add I/O accounting to msdos filesystem.

Suggested and reviewed by: kib

MFC 234026:
Expand locking around identification of filesystem mount point when
accounting for I/O counts at completion of I/O operation. Also switch
from using global devmtx to vnode mutex to reduce contention.

Suggested and reviewed by: kib

git-svn-id: svn://svn.freebsd.org/base/stable/9@234466 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f