MFC r319542,321003,321378
Eliminate duplication of the pmap and pv list unlock operations in pmap_enter() by implementing a single return path. Otherwise, the duplication will only increase with the upcoming support for psind == 1. Extract the innermost loop of pmap_remove() out into its own function, pmap_remove_ptes(). (This new function will also be used by an upcoming change to pmap_enter() that adds support for psind == 1 mappings.) Add support for pmap_enter(..., psind=1) to the amd64 pmap. In other words, add support for explicitly requesting that pmap_enter() create a 2MB page mapping. (Essentially, this feature allows the machine-independent layer to create superpage mappings preemptively, and not wait for automatic promotion to occur.) Export pmap_ps_enabled() to the machine-independent layer. Add a flag to pmap_pv_insert_pde() that specifies whether it should fail or reclaim a PV entry when one is not available. Refactor pmap_enter_pde() into two functions, one by the same name, that is a general-purpose function for creating PDE PG_PS mappings, and another, pmap_enter_2mpage(), that is used to prefault 2MB read- and/or execute-only mappings for execve(2), mmap(2), and shmat(2).