When vm_map_find(find_space = VMFS_OPTIMAL_SPACE) fails to find space, a
second scan of the address space with find_space = VMFS_ANY_SPACE is
performed. Previously, vm_map_find() released and reacquired the map lock
between the first and second scans. However, there is no compelling
reason to do so. This revision modifies vm_map_find() to retain the map
lock.

Reviewed by: jhb, kib, markj
MFC after: 1 week
X-Differential Revision: https://reviews.freebsd.org/D13155