FatPkg/EnhancedFatDxe Fix Double Cluster Allocation

FatPkg/EnhancedFatDxe Fix Double Cluster Allocation

This is a fix for a double cluster allocation when the disk is full.
The double allocation happens because FatGrowEof calls
FatAllocateCluster without immediately marking the each returned
cluster as allocated. The fix is to move the FatSetFatEntry call
inside the loop.
I've also include some improvements to the sanity checks that I added
while tracking this down. They are optional.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Herbie Robinson <Herbie.Robinson@stratus.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>