smack_common.sh: Cleanup

* On some systems 'smackfs' mounts aren't listed in the output from 'df',
  so we switch to the '/proc/mounts' instead.

* Use 'tst_brkm'.

* Use '$()' instead of '``'.

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
1 file changed