mirror: Don't call job_pause_point() under graph lock

mirror: Don't call job_pause_point() under graph lock

Calling job_pause_point() while holding the graph reader lock
potentially results in a deadlock: bdrv_graph_wrlock() first drains
everything, including the mirror job, which pauses it. The job is only
unpaused at the end of the drain section, which is when the graph writer
lock has been successfully taken. However, if the job happens to be
paused at a pause point where it still holds the reader lock, the writer
lock can't be taken as long as the job is still paused.

Mark job_pause_point() as GRAPH_UNLOCKED and fix mirror accordingly.

Cc: qemu-stable@nongnu.org
Buglink: https://issues.redhat.com/browse/RHEL-28125
Fixes: 004915a96a7a ("block: Protect bs->backing with graph_lock")
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20240313153000.33121-1-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>