Moves all buffer_head-related stuff out of linux/fs.h and into
linux/buffer_head.h.  buffer_head.h is currently included at the very
end of fs.h.  So it is possible to include buffer_head directly from
all .c files and remove this nested include.

Also rationalises all the set_buffer_foo() and mark_buffer_bar()
functions.  We have:

	set_buffer_foo(bh)
	clear_buffer_foo(bh)
	buffer_foo(bh)

and, in some cases, where needed:

	test_set_buffer_foo(bh)
	test_clear_buffer_foo(bh)

And that's it.

BUFFER_FNS() and TAS_BUFFER_FNS() macros generate all the above real
inline functions.  Normally not a big fan of cpp abuse, but in this
case it fits.  These function-generating macros are available to
filesystems to expand their own b_state functions.  JBD uses this in
one case.