shiftfs: uid/gid shifting bind mount

This allows any subtree to be uid/gid shifted and bound elsewhere.  It
does this by operating simlarly to overlayfs.  Its primary use is for
shifting the underlying uids of filesystems used to support
unpriviliged (uid shifted) containers.  The usual use case here is
that the container is operating with an uid shifted unprivileged root
but sometimes needs to make use of or work with a filesystem image
that has root at real uid 0.

The mechanism is to allow any subordinate mount namespace to mount a
shiftfs filesystem (by marking it FS_USERNS_MOUNT) but only allowing
it to mount marked subtrees (using the -o mark option as root).  Once
mounted, the subtree is mapped via the super block user namespace so
that the interior ids of the mounting user namespace are the ids
written to the filesystem.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

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v3 - update to 4.14 (d_real changes)
v1 - based on original shiftfs with uid mappings now done via s_user_ns
v2 - fix revalidation of dentries
     add inode aliasing
4 files changed