fsverity: explicitly check for buffer overflow in build_merkle_tree()

fsverity: explicitly check for buffer overflow in build_merkle_tree()

The new Merkle tree construction algorithm is a bit fragile in that it
may overflow the 'root_hash' array if the tree actually generated does
not match the calculated tree parameters.

This should never happen unless there is a filesystem bug that allows
the file size to change despite deny_write_access(), or a bug in the
Merkle tree logic itself.  Regardless, it's fairly easy to check for
buffer overflow here, so let's do so.

This is a robustness improvement only; this case is not currently known
to be reachable.  I've added a Fixes tag anyway, since I recommend that
this be included in kernels that have the mentioned commit.

Fixes: 56124d6c87fd ("fsverity: support enabling with tree block size < PAGE_SIZE")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230328041505.110162-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>