xfstests: use native definition of O_DIRECT flag

xfstests: use native definition of O_DIRECT flag

The definition of O_DIRECT in src/trunc.c causes xfstest 125 to fail
when run on a Pandaboard.  On ARM, the value used (0x040000) is
O_DIRECTORY rather than O_DIRECT as it is on x86.  Prefer the platform's
native definition of O_DIRECT supplied by fcntl.h if available.  Also,
fix a couple of error messages to properly reflect their context.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>