runltp: fix bad variable name error

Fix bad variable in runltp script for DEFAULT_FILE_NAME_GENERATION_TIME.
When running LTP at different language OS (chinese zh_CN), the command
"date + %b" will get output like " 4yue" that breaks the script becasue
it contains a space.

This commit changes the %b to %m and also adds quotes around the
variable.

BEWARE: This changes the default runltp logfile name, the month in the
        start date is now number instead of the abbreviation.

Signed-off-by: look <eeslook@163.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>