stream_smb: locking to bypass libsmbclient issues

Thread-unsafe libsmbclient is likely to crash when used simultaneously
across threads. For example, by:

    mpv "smb://...mkv" --sub-file "smb://...srt"

Work around this by locking all calls to this library under a single
global mutex. This is a temporary solution. When the libsmbclient bug
[1] is fixed, switch to the code from branch [2] which uses a new api to
create per-thread smb contexts. Fixes [3].

[1] https://bugzilla.samba.org/show_bug.cgi?id=11413
[2] https://github.com/orbisvicis/mpv/tree/smbclient_threaded_api
[3] https://github.com/mpv-player/mpv/issues/5936