9p: fix new mount API cache option handling

9p: fix new mount API cache option handling

After commit 4eb3117888a92, 9p needs to be able to accept numerical
cache= mount options as well as the string "shortcuts" because the option
is printed numerically in /proc/mounts rather than by string. This was
missed in the mount API conversion, which used an enum for the shortcuts
and therefore could not handle a numeric equivalent as an argument
to the cache option.

Fix this by removing the enum and reverting to the slightly more
open-coded option handling for Opt_cache, with the reinstated
get_cache_mode() helper.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Message-ID: <48cdeec9-5bb9-4c7a-a203-39bb8e0ef443@redhat.com>
Tested-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>