nvme: fix command effects log lifetime for multipath heads

In the Linux kernel, the following vulnerability has been resolved:

nvme: fix command effects log lifetime for multipath heads

KASAN reported a use-after-free when an I/O passthrough command was sent
through a multipath namespace head after the controller path that first
created the head had been removed:

BUG: KASAN: slab-use-after-free in nvme_command_effects+0x192/0x200 [nvme_core]
Read of size 4 at addr ffff888141b14400 by task nvme/19811
nvme_command_effects+0x192/0x200 [nvme_core]
nvme_cmd_allowed+0x7e/0x1b0 [nvme_core]
nvme_user_cmd.constprop.0+0x1b5/0x450 [nvme_core]
nvme_ns_head_chr_ioctl+0xf4/0x2a0 [nvme_core]

Move the log cache to the subsystem, with one entry per command set. Both
controllers and namespace heads hold subsystem references, keeping their
log pointers valid until subsystem release.