filesystem freeze: implement generic freeze feature

filesystem freeze: implement generic freeze feature

The ioctls for the generic freeze feature are below.
o Freeze the filesystem
  int ioctl(int fd, int FIFREEZE, arg)
    fd: The file descriptor of the mountpoint
    FIFREEZE: request code for the freeze
    arg: Ignored
    Return value: 0 if the operation succeeds. Otherwise, -1

o Unfreeze the filesystem
  int ioctl(int fd, int FITHAW, arg)
    fd: The file descriptor of the mountpoint
    FITHAW: request code for unfreeze
    arg: Ignored
    Return value: 0 if the operation succeeds. Otherwise, -1
    Error number: If the filesystem has already been unfrozen,
                  errno is set to EINVAL.

[[email protected]: fix CONFIG_BLOCK=n]
Signed-off-by: Takashi Sato <[email protected]>
Signed-off-by: Masayuki Hamaguchi <[email protected]>
Cc: <[email protected]>
Cc: <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dave Kleikamp <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Alasdair G Kergon <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>