sound: Fix lock order reversals in mseq_open()

Opening /dev/sequencer after a clean reboot yields:

lock order reversal: (sleepable after non-sleepable)
1st 0xfffffe004a2c2c08 seqflq (seqflq, sleep mutex) @ /mnt/src/sys/dev/sound/midi/sequencer.c:754
2nd 0xffffffff84197ed8 midistat lock (midistat lock, sx) @ /mnt/src/sys/dev/sound/midi/midi.c:1478
lock order seqflq -> midistat lock attempted at:
0xffffffff811c9029 at witness_checkorder+0x12b9
0xffffffff810f18a7 at _sx_xlock+0xf7
0xffffffff8417f992 at midimapper_open+0x22
0xffffffff84182770 at mseq_open+0xf0
0xffffffff80e3380f at devfs_open+0x30f
0xffffffff81b8b4b7 at VOP_OPEN_APV+0x57
0xffffffff812da1e7 at vn_open_vnode+0x397
0xffffffff812d96b3 at vn_open_cred+0xb23
0xffffffff812c2c6b at openatfp+0x52b
0xffffffff812c2711 at sys_openat+0x81
0xffffffff84110579 at filemon_wrapper_openat+0x19
0xffffffff81a223ae at amd64_syscall+0x39e
0xffffffff819dd0eb at fast_syscall_common+0xf8

Expose midistat_lock to midi/midi.c so that we can acquire the lock from
mseq_open() before we lock seq_lock, and introduce _locked variants of
midimapper_open() and midimapper_fetch_synth().

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, emaste
Differential Revision: https://reviews.freebsd.org/D45770