dm block manager: make block locking optional

The block manager's locking is useful for catching cycles that may
result from certain btree metadata corruption.  But in general it serves
as a developer tool to catch bugs in code.  Unless you're finding that
DM thin provisioning is hanging due to infinite loops within the block
manager's access to btree nodes you can safely disable this feature.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de> # do/while(0) macro fix
Signed-off-by: Mike Snitzer <snitzer@redhat.com>