kernel/fork.c: add comment about usage of CLONE_FS flags and namespaces

kernel/fork.c: add comment about usage of CLONE_FS flags and namespaces

All other places that deals with namespaces have an explanation of why
the restriction is there.

The description added in this commit was based on commit e66eded8309e
("userns: Don't allow CLONE_NEWUSER | CLONE_FS").

Link: http://lkml.kernel.org/r/20171112151637.13258-1-marcos.souza.org@gmail.com
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>