util: Correctly handle short writes in m5 (read|exec)file

util: Correctly handle short writes in m5 (read|exec)file

The m5 tool has subcommands that writes a file to the simulated file
system. The implementation of this command currently doesn't check the
return value from write, which leads to compiler warnings and
potentially incorrect behavior. Add the necessary checks.

Change-Id: If558534d3245aa24cf15edf06bd0af4c6ba3908c
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2962
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>