Implement VFS read clustering for MSDOSFS.

The logic used in msdosfs_bmap() to loop calling pcbmap() comes from
FreeBSD and is not really efficient but it is good enough since it is
only called when generating I/O.

With this diff I get a 100% improvement when reading big files from a
crappy USB stick.

With this and bread_cluster(9) modified to not re-fetch B_CACHED buffers,
reading large contiguous files with chunk sizes of MAXPHYS is almost as
fast as physio(9) on the same device.

For a 'real world' example, when copying music files from a USB stick I
see a speed jump from 15MB/s on -current to 24Mb/s with this diff.

While here rename some 'lbn' variables into 'cn' to better reflect what
we're dealing with.

Tested by Mathieu, with support from deraadt@

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