Merge all MD sf_buf allocators into one MI, residing in kern/subr_sfbuf.c
The MD allocators were very common, however there were some minor
differencies. These differencies were all consolidated in the MI allocator,
under ifdefs. The defines from machine/vmparam.h turn on features required
for a particular machine. For details look in the comment in sys/sf_buf.h.

As result no MD code left in sys/*/*/vm_machdep.c. Some arches still have
machine/sf_buf.h, which is usually quite small.

Tested by:	glebius (i386), tuexen (arm32), kevlo (arm32)
Reviewed by:	kib
Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Return USB_ERR_INVAL if the eject method is not known.

PR:		145319
Submitted by:	rozhuk.im gmail.com