9p: convert to the new mount API

Convert 9p to the new mount API. This patch consolidates all parsing
parsing into fs/9p/v9fs.c, which stores all results into a filesystem
context which can be passed to the various transports as needed.

Some of the parsing helper functions such as get_cache_mode() can be
eliminated in favor of using the new mount API's enum param type,
for simplicity.
12 files changed