Signed-off-by: Hugh Dickins <hugh@veritas.com>

It seems eccentric to implement MAP_POPULATE only on PROT_NONE mappings:
do_mmap_pgoff is passing down prot, then sys_remap_file_pages verifies it's
not set.  I guess that's an oversight from when we realized that the prot arg
to sys_remap_file_pages was misdesigned.

There's another oddity whose heritage is harder for me to understand, so
please let me leave it to you: sys_remap_file_pages is declared as asmlinkage
in mm/fremap.c, but is the one syscall declared without asmlinkage in
include/linux/syscalls.h.