bcachefs: fix overflow in fiemap

filefrag (and potentially other utilities that call fiemap) sometimes
pass ULONG_MAX as the length.  fiemap_prep clamps excessively large
lengths - but the calculation of end can overflow if it occurs before
calling fiemap_prep.  When this happens, filefrag assumes it has read to
the end and exits.

Signed-off-by: Reed Riley <reed@riley.engineer>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>