Fix deadlock on concurrent flush at the same cache

Signed-off-by: Robert Baldyga <robert.baldyga@intel.com>

tests/functional/pyocf/types/shared.py

@@ -41,6 +41,7 @@ class OcfErrorCode(IntEnum):
     OCF_ERR_DIRTY_SHUTDOWN = auto()
     OCF_ERR_DIRTY_EXISTS = auto()
     OCF_ERR_FLUSHING_INTERRUPTED = auto()
+    OCF_ERR_FLUSH_IN_PROGRESS = auto()
     OCF_ERR_CANNOT_ADD_CORE_TO_POOL = auto()
     OCF_ERR_CACHE_IN_INCOMPLETE_STATE = auto()
     OCF_ERR_CORE_IN_INACTIVE_STATE = auto()