PAGE_SIZE and size_t are not unsigned longs on 32 bit, annoying...

also switch to atomic64_cmpxchg instead of cmpxchg() for
journal_seq_copy, as atomic64_cmpxchg has a fallback that uses spinlocks
for when it's not supported.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>