fs: Fixes a bug that limits volume size to 1MB

The FS needs to be allowed to choose the correct allocation unit
size based on volume size. Current code passes a fixed value to
the function that creates the FS. Passing 0 allows the FS to configure
the required au size.

Jira: ZEP-898
Change-Id: Ic43bfba8b8f8498f4db49db744743afad4b118cd
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>