bcachefs: fix seqmutex_relock()

We were grabbing the sequence number before unlock incremented it - fix
this by moving the increment to seqmutex_lock() (so the seqmutex_relock()
failure path skips the mutex_trylock()), and returning the sequence
number from unlock(), to make the API simpler and safer.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>