exfat: fix random stack corruption after get_block

In the Linux kernel, the following vulnerability has been resolved:

exfat: fix random stack corruption after get_block

When get_block is called with a buffer_head allocated on the stack, such

as do_mpage_readpage, stack corruption due to buffer_head UAF may occur in

the following race condition situation.

<CPU 0> <CPU 1>

mpage_read_folio

<<bh on stack>>

do_mpage_readpage

exfat_get_block

bh_read

__bh_read

get_bh(bh)

submit_bh

wait_on_buffer

...

end_buffer_read_sync

__end_buffer_read_notouch

unlock_buffer

<<keep going>>

...

...

...

...

<<bh is not valid out of mpage_read_folio>>

.

.

another_function

<<variable A on stack>>

put_bh(bh)

atomic_dec(bh->b_count)

* stack corruption here *

This patch returns -EAGAIN if a folio does not have buffers when bh_read

needs to be called. By doing this, the caller can fallback to functions

like block_read_full_folio(), create a buffer_head in the folio, and then

call get_block again.

Let's do not call bh_read() with on-stack buffer_head.