string: fix type use in strstr()

string: fix type use in strstr()

Using plain int for string lengths, while okay for all practical
purposes, is undesirable in a generic library function.

Take the opportunity and also move the function from being in the middle
of mem*() ones to the set of str*() ones, convert its loop from while()
to for(), and correct style.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>