VFS: prepare for changes to directory locking

These patches are part of some prep work to allow changes to directory
locking.
The short term goal is to invert the locking order between d_alloc_parallel() 
and inode_lock() so that d_alloc_parallel() can (only) be called without
holding inode_lock().  This particularly requires filesystems not to use 
d_drop() in the middle of an operation, which particularly requires the d_splice_alias()
change.

As it will require d_alloc_parallel() to NOT be called under the inode
lock, d_alloc_trylock() is added which CAN be used when failure is safe
(dcache priming in readdir).

I have other related patches which make changes to lookup_open() and
related code but I think it best to wait for the O_CREAT|O_DIRECTORY
patches to land first (unless it is decided that they won't land).

If these patches can land in a stable branch, I can start making changes
to various filesystems which depend on these.

Thanks,
NeilBrown

 [PATCH 1/4] VFS: fix various typos in documentation for
 [PATCH 2/4] VFS: enhance d_splice_alias() to handle in-lookup
 [PATCH 3/4] VFS: introduce d_alloc_trylock()
 [PATCH 4/4] VFS: add d_duplicate()