FatPkg/EnhancedFatDxe: Ensure traverse of subtasks is delete-safe

FatPkg/EnhancedFatDxe: Ensure traverse of subtasks is delete-safe

Within function FatQueueTask(), the traverse of FAT subtasks for
executing the disk read/write is not delete-safe.

For the below case:

FatDiskIo(): When non-blocking access, creates subtasks and creates
event (FatOnAccessComplete, NOTIFY level) when subtasks finish.

FatQueueTask(): Traverses the subtasks and submits them one by one at
Tpl lower than NOTIFY.

Disk R/W completes really quick.

FatOnAccessComplete(): Removes the finished subtask, causing the
traverse in FatQueueTask() broken.

This commits will refine the subtask traverse in FatQueueTask() to be
delete-safe.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>