os/LevelDBStore: handle deletion race when checking store size

This fixes the fix in 64774e5792f136df2bc78db686440fc2f3a7643f which mixed
up the return value and errno.

Fixes: #6550
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>