pstore: Change kmsg_bytes storage size to u32

pstore: Change kmsg_bytes storage size to u32

The types around kmsg_bytes were inconsistent. The global was unsigned
long, the argument to pstore_set_kmsg_bytes() was int, and the filesystem
option was u32. Given other internal limits, there's not much sense
in making a single pstore record larger than INT_MAX and it can't be
negative, so use u32 everywhere. Additionally, use READ/WRITE_ONCE and a
local variable in pstore_dump() to avoid kmsg_bytes changing during a
dump.

Link: https://lore.kernel.org/r/20250206191655.work.798-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>