msm: ADSPRPC: Fix to avoid race condition and use after free

Current mmap and munmap use same mutex but munmap_fd does not
use the same. This can introduce race condition between mmap
and munmap_fd. Also there is a use after free scenario in get_args
and init_process as only mmap_create is protected and the map can
be freed after this. Unifying mutex to avoid race condition.
Restricting munmap_fd only for persist bufs to avoid this.

Change-Id: I2adf631b1e61c2274a14a3645a5e555f5d248645
Acked-by: Ekansh Gupta <ekangupt@qti.qualcomm.com>
Signed-off-by: Mohammed Nayeem Ur Rahman <mohara@codeaurora.org>