btrfs: sanitize security_mnt_opts use

1) keeping a copy in btrfs_fs_info is completely pointless - we never
use it for anything.  Getting rid of that allows for simpler calling
conventions for setup_security_options() (caller is responsible for
freeing mnt_opts in all cases).

2) on remount we want to use ->sb_remount(), not ->sb_set_mnt_opts(),
same as we would if not for FS_BINARY_MOUNTDATA.  Behaviours *are*
close (in fact, selinux sb_set_mnt_opts() ought to punt to
sb_remount() in "already initialized" case), but let's handle
that uniformly.  And the only reason why the original btrfs changes
didn't go for security_sb_remount() in btrfs_remount() case is that
it hadn't been exported.  Let's export it for a while - it'll be
going away soon anyway.

Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>