Btrfs: fix "mutually exclusive op is running" error code

The error code that is returned in response to starting a mutually
exclusive operation when there is one already running got silently
changed from EINVAL to EINPROGRESS by 5ac00add.  Returning EINPROGRESS
to, say, add_dev, when rm_dev is running is misleading.  Furthermore,
the operation itself may want to use EINPROGRESS for other purposes.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>