Fix bug in get_per_page_niobufs() which always used the niobuf flags from

Fix bug in get_per_page_niobufs() which always used the niobuf flags from
the first niobuf (which had OBD_BRW_FROM_GRANT set) and not any subsequent
niobuf (which generally didn't have it set).  This confused the obdfilter
into thinking the client was claiming a write was from grant.  With the old
limit scheme that was fatal, but the credits scheme handled it easily and
just reported an ungranted write until I could track it down.

Fix accounting for truncated pages, hopefully.  Also paves the way for
grant expiry in the future.

Change "CDEBUG(0, ...)" to mean "don't print" (compiler should remove it
entirely) so that we can turn off debug messages without deleting them
from the code.  That was only used in one place to mean "always print".

Add /proc/*/OSC/*/cur_grant_bytes for debugging.

Remove some old cruft to make room for grant fields.

b=974