run_memctl_test.sh: replace tr + cut with awk

In RHEL6.5GA and RHEL7.0GA, running run_memctl_test.sh outputs
"/opt/ltp/testcases/bin/run_memctl_test.sh: line 87: [: -lt: unary
operator expected" error. This is because $MEM_AVAIL is empty here,
which is caused by command "tr -s [:space:]" because the string
[:space:] is interpreted as wildcard and evaluates to 'c' because a file
with this name exist in the installed testcases/bin/ directory.

Signed-off-by: Xing Gu <gux.fnst@cn.fujitsu.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>