bcachefs: remove duplicated assignment to variable offset_into_extent

bcachefs: remove duplicated assignment to variable offset_into_extent

Variable offset_into_extent is being assigned to zero and a few
statements later it is being re-assigned again to the save value.
The second assignment is redundant and can be removed. Cleans up
clang-scan build warning:

fs/bcachefs/io.c:2722:3: warning: Value stored to 'offset_into_extent'
is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>