fs: remove vfs_stat_set_lookup_flags

The function really obsfucates checking for valid flags and setting the
lookup flags.  The fact that it returns -EINVAL through and unsigned
return value, which is then used as boolean really doesn't help either.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>