mon: MonitorStore: check mkdir return value for errors

mon: MonitorStore: check mkdir return value for errors

CID 728081: Unchecked return value (CHECKED_RETURN)
At (5): Calling function "mkdir(dfn, 493U)" without checking return value.
CID 716853: Unchecked return value (CHECKED_RETURN)
At (2): Calling function "mkdir(fn, 493U)" without checking return value.
CID 716855: Unchecked return value (CHECKED_RETURN)
At (2): Calling function "mkdir(fn, 493U)" without checking return value.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>