[readdir] constify ->actor

arch/alpha/kernel/osf_sys.c

arch/parisc/hpux/fs.c

fs/compat.c

fs/exportfs/expfs.c

fs/gfs2/export.c

fs/nfsd/nfs4recover.c

fs/nfsd/vfs.c

fs/readdir.c

include/linux/fs.h