e2fsck: use size_t instead of int in string_copy()

e2fsck: use size_t instead of int in string_copy()

len argument in string_copy() is int, but it is used with malloc(),
strlen(), strncpy() and some callers use sizeof() to pass value in. So
it really ought to be size_t rather than int. Fix it.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>