btrfs: add helper to clear EXTENT_BUFFER_READING

We are clearing the bit and waking up any waiters in two different
places.  Factor that code out into a static helper function.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Tavian Barnes <tavianator@tavianator.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>