[PATCH -V2] Generic name to handle and open by handle syscalls

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

The below set of patches implement open by handle support using exportfs
operations. This allows user space application to map a file name to file 
handle and later open the file using handle. This should be usable
for userspace NFS [1] and 9P server [2]. XFS already support this with the ioctls
XFS_IOC_PATH_TO_HANDLE and XFS_IOC_OPEN_BY_HANDLE.

[1] http://nfs-ganesha.sourceforge.net/
[2] http://lists.gnu.org/archive/html/qemu-devel/2010-03/msg01087.html

Changes from v1:
a) handle size is now specified in bytes
b) returns -EOVERFLOW if the handle size is small
c) dropped open_handle syscall and added open_by_handle_at syscall
   open_by_handle_at takes mount_fd as the directory fd of the mount point
   containing the file
e) handle will only be unique in a given file system. So for an NFS server
   exporting multiple file system, NFS server will have to internally track the
   mount point to which a file handle belongs to. We should be able to do it much
   easily than expecting kernel to give a system wide unique file handle. System
   wide unique file handle would need much larger changes to the exportfs or VFS
   interface and I was not sure whether we really need to do that in the kernel or
   in the user space
f) open_handle_at now only check for DAC_OVERRIDE capability


Example program:
-------------
cc  -D_GNU_SOURCE  <src.c>
----------------
#include <stdio.h>
#include <stdlib.h>

#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>


struct file_handle {
        int handle_size;
        int handle_type;
        void *handle;
};
int main(int argc, char *argv[])
{
        int ret;
        int fd, dirfd;
        char buf[100];
        struct file_handle fh;
        fh.handle_type = 0;
        fh.handle = malloc(100);
        fh.handle_size = 100;
        errno  = 0;
        ret = syscall(338, argv[1], &fh);
        if (ret) {
                perror("Error:");
                exit(1);
        }
        dirfd = open("/", O_RDONLY|O_DIRECTORY);
        fd = syscall(339, dirfd, &fh, O_RDONLY);
        if (fd <= 0 ) {
                perror("Error:");
                exit(1);
        }
        memset(buf, 0 , 100);
        while (read(fd, buf, 100) > 0) {
                printf("%s", buf);
                memset(buf, 0 , 100);
        }
        return 0;
}