ext4: simplify ext4_readpage_limit()

Now that the implementation of FS_IOC_ENABLE_VERITY has changed to not
involve reading back Merkle tree blocks that were previously written,
there is no need for ext4_readpage_limit() to allow for this case.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20221223203638.41293-9-ebiggers@kernel.org