Make mmap(MAP_STACK) search for the available address space, similar
to !MAP_STACK mapping requests.  For MAP_STACK | MAP_FIXED, clear any
mappings which could previously exist in the used range.

For this, teach vm_map_find() and vm_map_fixed() to handle
MAP_STACK_GROWS_DOWN or _UP cow flags, by calling a new
vm_map_stack_locked() helper, which is factored out from
vm_map_stack().

The side effect of the change is that MAP_STACK started obeying
MAP_ALIGNMENT and MAP_32BIT flags.

Reported by:	rwatson
Reviewed by:	alc
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Change the nblock mutex, protecting the needsbuffer buffer deficit
flags, to rwlock.  Lock it in read mode when used from subroutines
called from buffer release code paths.

The needsbuffer is now updated using atomics, while read lock of
nblock prevents loosing the wakeups from bufspacewakeup() and
bufcountadd() in getnewbuf_bufd_help().

In several interesting loads, needsbuffer flags are never set, while
buffers are reused quickly.  This causes brelse() and bqrelse() from
different threads to content on the nblock.  Now they take nblock in
read mode, together with needsbuffer not needing an update, allowing
higher parallelism.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks