sound: Retire snd_mtx* wrappers
Do not create mutexes with snd_mtxcreate(). It doesn't provide any
value, plus it first allocates the mutex with malloc(9). Allocate
mutexes in the stack and use mtx_* functions directly instead of the
snd_mtx* wrappers.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D53855