newfs: nextnum should be a u_int32_t, not an int

The function that uses nextnum expects to return a u_int32_t, not a mere
int, so let's make nextnum a u_int32_t instead.

Note: retained current u_int32_t style, since the rest of the file uses
it.

Reviewed by: imp, mckusick
Pull Request: https://github.com/freebsd/freebsd-src/pull/734