bcachefs: Fix negative timespecs

This fixes two problems in the handling of negative times:

 • rem is signed, but the rem * c->sb.nsec_per_time_unit operation
   produced a bogus unsigned result, because s32 * u32 = u32.

 • The timespec was not normalized (it could contain more than a
   billion nanoseconds).

For example, { .tv_sec = -14245441, .tv_nsec = 750000000 }, after
being round tripped through timespec_to_bch2_time and then
bch2_time_to_timespec would come back as
{ .tv_sec = -14245440, .tv_nsec = 4044967296 } (more than 4 billion
nanoseconds).

Cc: stable@vger.kernel.org
Fixes: 595c1e9b ("bcachefs: Fix time handling")
Closes: https://github.com/koverstreet/bcachefs/issues/743


Co-developed-by: Erin Shepherd <erin.shepherd@e43.eu>
Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
Co-developed-by: Ryan Lahfa <ryan@lahfa.xyz>
Signed-off-by: Ryan Lahfa <ryan@lahfa.xyz>
Signed-off-by: Alyssa Ross <hi@alyssa.is>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>