Prepare for new export_operations interface (for filehandle lookup):

 - define d_splice_alias and d_alloc_anon.
 - define shrink_dcache_anon for removing anonymous dentries
 - modify d_move to work with anonymous dentries (IS_ROOT dentries)
 - modify d_find_alias to avoid anonymous dentries where possible
   as d_splice_alias and d_alloc_anon use this
 - put in place infrastructure for s_anon allocation and cleaning
 - replace a piece of code that is in nfsfh, reiserfs and fat
   with a call to d_alloc_anon
 - Rename DCACHE_NFSD_DISCONNECTED to DCACHE_DISCONNECTED
 - Add documentation at Documentation/filesystems/Exporting