Since the only caller to vm_map_splay is vm_map_lookup_entry, move the implementation of vm_map_splay into vm_map_lookup_entry.
vm_map_lookup_entry returns the greatest entry less than or equal to a given address, but in many cases the caller wants the least entry greater than or equal to the address and uses the next pointer to get to it. Provide an alternative interface to lookup, vm_map_lookup_entry_greq, to provide the latter behavior, and let callers use one or the other rather than having them use the next pointer after a lookup miss to get what they really want.
In vm_map_growstack, the caller wants an entry that includes a given address, and either the preceding or next entry depending on the value of eflags in the first entry. Incorporate that behavior into vm_map_lookup_helper, the function that implements all of these lookups.
Eliminate some temporary variables used with vm_map_lookup_entry, but inessential.
The primary objective of this change is to reduce the use of the prev and next fields of vm_map_entries. Once this patch is applied, there will be only one use of the prev field used for any purpose other than maintaining the value of the prev field.