posix: mutex: use k_mutex instead of posix_mutex

The internal representation of `pthread_mutex_t`,
`struct posix_mutex`, is basically a clone of `struct k_mutex`
but without the benefit of being able to use all of the
existing `k_mutex_*()` suite of functions.

The first step in the right direction was switching
the external representation of `pthread_mutex_t` to a simple
`int`. Let's take the next step in the right direction, which
is getting rid of `struct posix_mutex`.

The only significant difference between `struct k_mutex` and
`struct posix_mutex` is that the latter needs a `type` field.

Since there were a fixed number of `struct posix_mutex`, we
can just externalize the `type` field and reuse
`struct k_mutex` as-is.

For now, let's keep this change as a simple type
substitution. Eventually, we should be able to fully switch
to Zephyr API internally.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>