vold: don't crash when formatting unformatted volume

When formatting unformatting volumes, fstype will be NULL causing
a crash in strcmp. Set it to "vfat" to avoid the crash.

Change-Id: I3876ea2a6d91e78f579537fd6ea3d030fb18ee7b
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_system_vold into temp