CIFS: cleanup min_t() cast in cifs_read()

Smatch complains that the cast to "int" in min_t() changes very large
values of current_read_size into negative values and so min_t()
could return the wrong value.  I removed the const as well, as that
doesn't do anything here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <smfrench@gmail.com>