create common entry point for Rust commands

create common entry point for Rust commands

The bcachefs sub-commands that are implemented in Rust (completions,
list, and mount) had separate entrypoints and thus had some differences
in behavior.

This introduces a common entry point for the Rust sub-commands. This
reduces duplicate boilerplate code like parsing argv and setting up
logging, and will facilitate converting more sub-commands to Rust in
the future.

An immediate benefit is that this fixes an issue with `bcachefs list`
not reporting errors:

before:

$ bcachefs list /dev/typo
$ echo $?
0

after:

$ bcachefs list /dev/typo
ERROR - bcachefs_rust::cmd_list: Fatal error: "No such file or directory"
$ echo $?
1

Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>