Revert change made in base r171522
(https://svnweb.freebsd.org/base?view=revision&revision=304232)
converting clrbuf() (which clears the entire buffer) to vfs_bio_clrbuf()
(which clears only the new pages that have been added to the buffer).

Failure to properly remove pages from the buffer cache can make
pages that appear not to need clearing to actually have bad random
data in them. See for example base r304232
(https://svnweb.freebsd.org/base?view=revision&revision=304232)
which noted the need to set B_INVAL and B_NOCACHE as well as clear
the B_CACHE flag before calling brelse() to release the buffer.

Rather than trying to find all the incomplete brelse() calls, it
is simpler, though more slightly expensive, to simply clear the
entire buffer when it is newly allocated.

PR: 213507
Submitted by: Damjan Jovanovic
Reviewed by: kib