btrfs: add RENAME_NOREPLACE

RENAME_NOREPLACE is trivial to implement for most filesystems: switch over
to ->rename2() and check for the supported flags.  The rest is done by the
VFS.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Chris Mason <clm@fb.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>