ALSA: seq: Use RCU for the UMP client output substream

The UMP sequencer client protects its output rawmidi file (out_rfile)
with an rwlock (output_lock).  seq_ump_process_event(), the port's
event_input callback, reads out_rfile.output under read_lock on every
delivered UMP event, while the open/close paths (serialized by
ump->open_mutex) publish and clear out_rfile under write_lock.

Output is opened/closed only on the subscribe/use lifecycle while
delivery happens per event, so this is another read-mostly hot path.
Convert it to RCU and drop the rwlock.  out_rfile is an embedded struct
rather than a pointer, so instead of restructuring it, add an
RCU-protected shadow of the substream (out_substream) for the reader;
out_rfile itself becomes writer-only state accessed solely under
open_mutex.  The reader now runs lock-free under rcu_read_lock() via
rcu_dereference(), and open publishes the substream with
rcu_assign_pointer().

On close the substream is cleared with rcu_assign_pointer(NULL) and the
rawmidi is released only after synchronize_rcu(), so no reader in the
delivery path can still be writing to the substream when
snd_rawmidi_kernel_release() runs.

Dropping write_lock_irqsave() from the writers is safe: they run in
process context under open_mutex, and the sole atomic reader now uses
RCU, which is IRQ-safe.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260810133711.42483-6-tiwai@suse.de

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