remoteproc: core: Fetch the auto-boot firmware only once

Auto-boot for an always-on remote processor registers an asynchronous
request_firmware_nowait() whose callback, rproc_auto_boot_callback(),
calls rproc_boot() first and only releases the fetched image at the
very end of the callback.  rproc_boot() does not take this image as an
argument: it fetches the very same image again with a synchronous
request_firmware(), so every auto-boot reads the firmware image twice
and allocates the buffer twice; on kernels with the sysfs fallback
enabled, the uevent round-trip is repeated as well.

The asynchronous request exists only so that rproc_add() does not
block on the filesystem read; the image it fetches is never used.
The core already defers rproc_boot() to a worker for detached
processors (attach_work).  Reuse that worker for offline processors
too and drop the asynchronous firmware request: rproc_boot() fetches
the image exactly once and dispatches between a firmware boot and
an attach based on the processor state.  The work is renamed to
boot_work to match its widened role.

commit 400e64df6b23 ("remoteproc: add framework for controlling remote
processors") noted back in 2011 that "we must wait until it completes
before we try to unregister the device".  rproc_del() now does exactly
that: it waits for the boot work with cancel_work_sync(), which also
closes the theoretical window in which a pending attach work could
outlive the rproc instance.

The changed fallback behaviour only matters for legacy
configurations.  udev dropped its userspace firmware loader back in
2014, as recorded in
Documentation/driver-api/firmware/fallback-mechanisms.rst, and the
kernel has documented since commit 02c399306826 ("firmware_loader:
enhance Kconfig documentation over FW_LOADER") that "Linux no longer
relies on or uses a fallback mechanism in userspace".  Auto-boot now
uses the same synchronous fallback semantics as every other explicit
boot source (sysfs, cdev).

Signed-off-by: Yonghao Zhang <hyz3367@gmail.com>
Link: https://lore.kernel.org/r/20261001081056.38293-1-hyz3367@gmail.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

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