new helper: vm_mmap_pgoff()

take it to mm/util.c, convert vm_mmap() to use of that one and
take it to mm/util.c as well, convert both sys_mmap_pgoff() to
use of vm_mmap_pgoff()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc mm/internal.h
Simple merge

diff --cc mm/mmap.c
Simple merge

diff --cc mm/nommu.c
Simple merge

diff --cc mm/util.c
Simple merge