TFTP : tftp fix for full volume case

TFTP : tftp fix for full volume case

Issue :
When storage media is full, tftp was resulting in ASSERT
MdeModulePkg/Core/Dxe/Mem/Page.c, because number of pages
was zero.

Reason:
While doing tftp, function call ShellWriteFile was modifying
FileSize variable. In case of full disk it was coming out to be
Zero.

Fix:
Storage the original filesize in local variable, and use this
variable while freeing the pages.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Udit Kumar <udit.kumar@nxp.com>
Signed-off-by: Meenakshi Aggarwal <meenakshi.aggarwal@nxp.com>
Signed-off-by: Vabhav Sharma <vabhav.sharma@nxp.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>