Add the definition of maxbcachebuf to sys/buf.h.

r320070 removed the definition of maxbcachebuf from sys/param.h to
fix the build for arm.
This patch adds the definition of maxbcachebuf to sys/buf.h, which
should be ok, since sys/buf.h is not being included in arm/arm/elf_note.S.

Suggested by: kib
MFC after: 2 weeks