Add bio_ops->io_checkread and io_checkwrite - a read and write pre-check

Add bio_ops->io_checkread and io_checkwrite - a read and write pre-check
which gives HAMMER a chance to set B_LOCKED if the kernel wants to write out
a passively held buffer.

Change B_LOCKED semantics slightly.  B_LOCKED buffers will not be written
until B_LOCKED is cleared.  This allows HAMMER to hold off B_DELWRI writes
on passively held buffers.