fs/9p: only translate RWX permissions for plain 9P2000

mainline inclusion
from mainline-v6.9-rc5
commit cd25e15e57e68a6b18dc9323047fe9c68b99290b
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9UNVB
CVE: CVE-2024-36964

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cd25e15e57e68a6b18dc9323047fe9c68b99290b



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Garbage in plain 9P2000's perm bits is allowed through, which causes it
to be able to set (among others) the suid bit. This was presumably not
the intent since the unix extended bits are handled explicitly and
conditionally on .u.

Signed-off-by: Joakim Sindholt <opensource@zhasha.com>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>
Signed-off-by: Ye Bin <yebin@huaweicloud.com>