Implement vfs clustering for msdosfs.

This gives a very large speedup for small block sizes (in my tests,
about 5 times for write and 3 times for read with a block size of 512,
if clustering is possible) and a moderate speedup for the moderatatly
large block sizes that should be used on non-small media (4K is the
best size in most cases, and the speedup for that is about 1.3 times
for write and 1.2 times for read).  mmap() should benefit from clustering
like read()/write(), but the current implementation of vm only supports
clustering (at least for getpages) if the fs block size is >= PAGE SIZE.

msdosfs is now only slightly slower than ffs with soft updates for
writing and slightly faster for reading when both use their best block
sizes.  Writing is slower for msdosfs because of more sync writes.
Reading is faster for msdosfs because indirect blocks interfere with
clustering in ffs.

The changes in msdosfs_read() and msdosfs_write() are simpler merges
of corresponding code in ffs (after fixing some style bugs in ffs).
msdosfs_bmap() needs fs-specific code.  This implementation loops
calling a lower level bmap function to do the hard parts.  This is a
bit inefficient, but is efficient enough since msdsfs_bmap() is only
called when there is physical i/o to do.

Approved by:	re (hrs)

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