btrfs: collapse wait_on_state() to its caller wait_extent_bit()

The wait_on_state() function is very short and has a single caller, which
is wait_extent_bit(), so remove the function and put its code into the
caller.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>