We shouldn't be returning success from this function without also
filling in the return values ctx and ctxlen.

Note currently this doesn't appear to cause bugs since the only
inode_getsecctx caller I can find is fs/sysfs/inode.c, which only calls
this if security_inode_setsecurity succeeds.  Assuming
security_inode_setsecurity is set to cap_inode_setsecurity whenever
inode_getsecctx is set to cap_inode_getsecctx, this function can never
actually called.

So I noticed this only because the server labeled NFS patches add a real
caller.

Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>