devpts: fix double-free on mount failure

devpts_kill_sb() is called even if devpts_fill_super() fails;
we should not do that kfree() in the latter, especially not
with ->s_fs_info left pointing to freed object.  Double kfree()
is a Bad Thing(tm)...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>