bcachefs: Fix shift by 64 in set_inc_field()

bcachefs: Fix shift by 64 in set_inc_field()

UBSAN was complaining about a shift by 64 in set_inc_field().

This only happened when the value being shifted was 0, so in theory
should be harmless - a shift by 64 (or register width) should logically
give a result of 0, but CPUs will in practice leave the input unchanged
when the number of bits to shift by wraps - and since our input here is
0, the output is still what we want.

But, it's still undefined behaviour and we need our UBSAN output to be
clean, so it needs to be fixed.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>