Synchronize extent_grow_next accesses.
Synchronize extent_grow_next accesses.

This should have been part of 411697adcda2fd75e135cdcdafb95f2bd295dc7f
(Use exponential series to size extents.), which introduced
extent_grow_next.

wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc