Fix parser bug

Fix parser bug

	If the option parser sees a ':', it automatically assumes that
you meant a range.  It will then try to parse the second half of the
option.  This isn't just my external: option, it happens to mem: as
well.  That is:

    1) Option mem=mmaphuge:/dev/shm/foo
    2) handle_option() notices the ':' and stores off ptr2.
    3) __handle_option() corectly sees the option "mem" and maps
       "mmaphuge" to MEM_MMAPHUGE.  It then calls str_mem_cb(), which
       stores off the "/dev/shm/foo" after the ':'.  Still working
       great.
    4) Control returns to handle_option(), which sees that ptr2 is
       non-NULL.  It calls __handle_option() again, pointing to the
       string after the ':'.
    5) __handle_option() prints an error, as "/dev/shm/foo" is not a
       known option.

Fix it by breaking out early for the 2nd parse round if the option
type is a string.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>