btrfs: assert we have a write lock when removing and replacing extent maps

Removing or replacing an extent map requires holding a write lock on the
extent map's tree. We currently do that everywhere, except in one of the
self tests, where it's harmless since there's no concurrency.

In order to catch possible races in the future, assert that we are holding
a write lock on the extent map tree before removing or replacing an extent
map in the tree, and update the self test to obtain a write lock before
removing extent maps.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>