MINOR: fd: move a few read-mostly variables to their own section

MINOR: fd: move a few read-mostly variables to their own section

Some pointer to arrays such as fdtab, fdinfo, polled_mask etc are never
written to at run time but are used a lot. fdtab accesses appear a lot in
perf top because ha_used_fds is in the same cache line and is modified
all the time. This patch moves all these read-mostly variables to the
read_mostly section when defined. This way their cache lines will be
able to remain in shared state in all CPU caches.