`FS.cacheExpiry` is accessed through sync/atomic.
According to the documentation, "On ARM, 386, and 32-bit MIPS, it is
the caller's responsibility to arrange for 64-bit alignment of 64-bit
words accessed atomically. The first word in a variable or in an
allocated struct, array, or slice can be relied upon to be 64-bit
aligned."
Before commit 1d2fe0d8564bc679ece166c24b24e6fe7dc1455c this field was
aligned, but then a new field was added to the structure, causing the
test suite to panic on linux/386.
No other field is used with sync/atomic, so `cacheExpiry` can just be
placed at the beginning of the stuct to ensure it is always aligned.
67fd60275a