fd: streamline fget_unlocked

clang has the unfortunate property of paying little attention to prediction
hints when faced with a loop spanning the majority of the rotuine.

In particular fget_unlocked has an unlikely corner case where it starts almost
from scratch. Faced with this clang generates a maze of taken jumps, whereas
gcc produces jump-free code (in the expected case).

Work around the problem by providing a variant which only tries once and
resorts to calling the original code if anything goes wrong.

While here note that the 'seq' parameter is almost never passed, thus the
seldom users are redirected to call it directly.