[patch 00/10] (resend) mount ownership and unprivileged mount syscall

[Miklos Szeredi <miklos@szeredi.hu>]

This patchset adds support for keeping mount ownership information in
the kernel, and allow unprivileged mount(2) and umount(2) in certain
cases.

This can be useful for the following reasons:

- mount(8) can store ownership ("user=XY" option) in the kernel
  instead, or in addition to storing it in /etc/mtab.  For example if
  private namespaces are used with mount propagations /etc/mtab
  becomes unworkable, but using /proc/mounts works fine

- fuse won't need a special suid-root mount/umount utility.  Plain
  umount(8) can easily be made to work with unprivileged fuse mounts

- users can use bind mounts without having to pre-configure them in
  /etc/fstab

Unprivileged mounts are restricted to private namespaces created with
a special clone flag.

Changes from the previous submission:

 - add namespace flag for allowing user mounts
 - add clone flag to set above namespace flag
 - make max number of user mounts default to 1024, since now the
   namespace flag will prevent user mounts by default

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