btrfs: remove trivial locking wrappers of tree mod log

[ Upstream commit b1a09f1ec540408abf3a50d15dff5d9506932693 ]

The wrappers are trivial and do not bring any extra value on top of the
plain locking primitives.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>