vm_page: use lookup_ge in grab_valid_iter()

To improve performance slightly, use vm_radix_iter_lookup_ge to skip
over several missing pages at once in grab_valid_iter().

Fix a case in which the function could return with the iterator
argument not reset after a lock release/reacquire.

Reviewed by: alc, markj
Differential Revision: https://reviews.freebsd.org/D50601