ShellPkg: Add FileSize member to shell memory file structure.

ShellPkg: Add FileSize member to shell memory file structure.

The shell uses the memory file structure to manage temporary files in
memory that support piping of output from one command into the the
input of another command.  The BufferSize member is the size of the
internal buffer, not the size of the data that was written to the
file. So, it was possible to read beyond the EOF of these files as
reads used BufferSize. Now FileSize tracks the actual size of these
files (the number of bytes written, not the number of bytes available
in the buffer), and the reads use this member.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jim Dailey <jim_dailey@dell.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>