libxfs: don't pass negative errnos to strerror()

libxfs: don't pass negative errnos to strerror()

The error negation work in 12b5319 tripped up a little bit
when we're reporting errors via strerror().  By negating
the error before passing it to strerror, we get i.e.

   mkfs.xfs: pwrite64 failed: Unknown error -22

Keep the error positive, but return -error, just as we
do in the else clauses in these functions.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>