Fix up r300870

The sys/types.h fix I proposed was only tested with zfs(4), not with
libzpool, which is where the build failure actually existed

Remove vm/vm_pageout.h from arc.c and zfs_vnops.c because they're both
unneeded

MFC after: 1 week
X-MFC with: r300865, r300870
In collaboration with: kib
Submitted by: alc
Sponsored by: EMC / Isilon Storage Division