BUF/BIO - MPSAFE work on core buffer cache routines.

BUF/BIO - MPSAFE work on core buffer cache routines.

Do initial MPSAFE work on getblk(), bread(), brelse(), bqrelse(),
biowait(), and friends.  Use get_mplock()/rel_mplock() in areas not
yet converted.

Add a flags argument to findblk() and have it return a locked buffer
cache buffer by default.  Callers may request that the buffer be
locked non-blocking, or that it not be locked at all.

Adjust all callers of findblk() for those cases where an unlocked
buffer is desired to lock and re-check the returned buffer's parameters,
since otherwise it can change out from under the caller whether a critical
section is used or not (for SMP).