CLEANUP: ring: rearrange the wait loop in ring_write()

CLEANUP: ring: rearrange the wait loop in ring_write()

The loop is constructed in a complicated way with a single break
statement in the middle and many continue statements everywhere,
making it hard to better factor between variants. Let's first
reorganize it so as to make it easier to escape when the ring
tail lock is obtained. The sequence of instrucitons remains the
same, it's only better organized.