core: fix and simplify generic_getfssec()

core: fix and simplify generic_getfssec()

Both simplify and fix generic_getfssec().  The previous version would
occasionally request the same extent twice from the underlying layer.
This is not a big deal for ext2fs for example, but is *very* expensive
on FAT, where any out-of-order next_extent() call means walking the
FAT from the beginning.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>