Unionfs: reduce number of whiteouts by deleting all instances of files

Unionfs: reduce number of whiteouts by deleting all instances of files

Optimize the unlinking of non-dir objects in unionfs by deleting all
possible lower inode objects from all writable lower branches.  This may
consume a bit more processing, but on average reduces overall inode
consumption and further saves a lot by reducing the total number of
whiteouts needed.  We create a whiteout now only if we could not delete all
lower objects, or if one of the lower branches was explicitly marked
read-only.

Signed-off-by: Himanshu Kanda <hkanda@cs.sunysb.edu>
Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>