bcachefs: fix stack corruption

bcachefs: fix stack corruption

When a bkey_on_stack is passed to bch_read_indirect_extent, there is no
guarantee that it will be big enough to hold the bkey. And
bch_read_indirect_extent is not aware of bkey_on_stack to call realloc
on it. This cause a stack corruption.

This commit makes bch_read_indirect_extent aware of bkey_on_stack so it
can call realloc when appropriate.

Tested-by: Yuxuan Shui <yshuiv7@gmail.com>
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>