Some compilers complain when comparing signed and unsigned. romio321
was doing just this. The check is meant to check whether a size (which
is an ADIO_Offset-- a signed number) will work with memcpy which takes
a size_t. To silence the warning I added a new type (ADIO_Size) which
is an unsigned type and cast the ADIO_Offset to this new type.
Fixes#5951
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>