ext4: fix potential race between online resizing and write operations

commit 1d0c3924a92e69bfa91163bda83c12a994b4d106 upstream.

During an online resize an array of pointers to buffer heads gets
replaced so it can get enlarged.  If there is a racing block
allocation or deallocation which uses the old array, and the old array
has gotten reused this can lead to a GPF or some other random kernel
memory getting modified.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=206443
Link: https://lore.kernel.org/r/20200221053458.730016-2-tytso@mit.edu


Reported-by: Suraj Jitindar Singh <surajjs@amazon.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org # 4.4.x
Cc: stable@kernel.org # 4.9.x
Signed-off-by: Sasha Levin <sashal@kernel.org>