xfs/195: Use XFSDUMP_PROG instead of xfsdump directly

xfs/195: Use XFSDUMP_PROG instead of xfsdump directly

Use appropriate environment variable (XFSDUMP_PROG) instead of
hardcoding 'xfsdump' name. Also make the test fail graciously when
xfsdump isn't installed.

We also make XFSDUMP_PROG be equal to empty string instead of -e when
xfsdump isn't installed. The changes require some tweaking to output
filtering since full command path now appears in the output.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>