vfs: add RENAME_WHITEOUT

This adds a new RENAME_WHITEOUT flag.  This flag makes rename() create a
whiteout of source.  The whiteout creation is atomic relative to the
rename.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

diff --cc fs/namei.c
Simple merge

diff --cc include/uapi/linux/fs.h
Simple merge