[BUG] ALSA: core: seq: a possible double-lock bug in snd_seq_midisynth_remove()

[Jia-Ju Bai <baijiaju1990@xxxxxxxxx>]

In snd_seq_midisynth_remove(), there is a possible double-lock bug:
snd_seq_midisynth_remove()
    mutex_lock(&register_mutex); -- line 421
    snd_seq_delete_kernel_client() --- line 436
        seq_free_client() -- line 2244
            mutex_lock(&register_mutex); -- line 294

This bug is found by a static analysis tool STCheck written by us.

I do not know how to correctly fix this bug, so I only report it.
A possible fix is to release the mutex lock before calling 
seq_free_client() in snd_seq_delete_kernel_client() and then acquiring 
the lock again after calling seq_free_client().


Best wishes,
Jia-Ju Bai
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