soc: aspeed: lpc-snoop: Fix usercopy overflow in snoop_file_read

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

soc: aspeed: lpc-snoop: Fix usercopy overflow in snoop_file_read

put_fifo_with_discard() acts as both producer and consumer on the kfifo:
it calls kfifo_skip() (advances out) and kfifo_put() (advances in) from
the IRQ handler without synchronizing with snoop_file_read(), which also
consumes via kfifo_to_user(). On SMP systems this concurrent access can
leave (in - out) larger than the ring buffer, so __kfifo_to_user()'s clamp
to (in - out) is ineffective and kfifo_copy_to_user() can attempt a
copy_to_user() past the kmalloc-2k backing store:

usercopy: Kernel memory exposure attempt detected from SLUB object
'kmalloc-2k' (offset 0, size 2049)!
kernel BUG at mm/usercopy.c!
Call trace:
usercopy_abort
__check_heap_object
__check_object_size
kfifo_copy_to_user
__kfifo_to_user
snoop_file_read
vfs_read

Serialize kfifo access with a per-channel spinlock shared between the
IRQ handler (producer) and the file reader (consumer). Annotate @fifo
with __guarded_by(&lock) and opt the driver into context analysis so the
compiler enforces that all fifo access holds the lock.