Delta should be greater than reserved size when check storage

Application can fill data partition full by writing data to
emulate storage, that would cause system run issue.
So, we should limit application write data to emulate storage
 by reserved size of sdcardfs file system.
But this case do not allow sdcardfs mount has reserved space.

Bug: 141520125
Change-Id: If3e3ecb1eb49e6111b7fc358ed2a293f250d8df2