vfs: canonicalize create mode in build_open_flags()

vfs: canonicalize create mode in build_open_flags()

commit e68726ff72cf7ba5e7d789857fcd9a75ca573f03 upstream.

Userspace can pass weird create mode in open(2) that we canonicalize to
"(mode & S_IALLUGO) | S_IFREG" in vfs_create().

The problem is that we use the uncanonicalized mode before calling vfs_create()
with unforseen consequences.

So do the canonicalization early in build_open_flags().

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>