mqueue: Explicitly capture the user namespace to send the notification to.

Stop relying on user->user_ns which is going away and instead capture
the user_namespace of the process we are supposed to notify.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
mqueue: lift mnt_want_write() outside ->i_mutex, clean up a bit

the way it abuses ->d_fsdata still needs to be killed, but that's
a separate story.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
switch mqueue syscalls to fget_light()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
switch simple cases of fget_light to fdget

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