Verify and maintain fanout invariants.

Verify and maintain fanout invariants.

This somewhat long patch calls various invariant-checking (debugging)
functions in all places where the fanout invariants should hold.  The three
invariant-checking functions, __unionfs_check_{inode,dentry,file}, perform
exhaustive sanity checking on the fan-out of various Unionfs objects.  We
check that no lower objects exist outside the start/end branch range; that
all objects within are non-NULL (with some allowed exceptions); that for
every lower file there's a lower dentry+inode; that the start/end ranges
match for all corresponding lower objects; that open files/symlinks have
only one lower objects, but directories can have several; and more.

The rest of this patch actually fixes many places where these invariants did
not hold, which could lead to bugs or corruptions under heavy loads,
multi-threaded workloads, dynamic branch-management, and mmap operations.
Most of the bugs related to actions involving copyups and whiteouts.  With
these fixes, the entire Unionfs regression suite passes without a single
invariant violated.