fs/9p: Consolidate file operations and add readahead and writeback

We had 3 different sets of file operations across 2 different protocol
variants differentiated by cache which really only changed 3
functions.  But the real problem is that certain file modes, mount
options, and other factors weren't being considered when we
decided whether or not to use caches.

This consolidates all the operations and switches
to conditionals within a common set to decide whether or not
to do different aspects of caching.

Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>
Reviewed-by: Dominique Martinet <asmadeus@codewreck.org>