fscrypt: compute max_lblk_bits from s_maxbytes and block size

For a given filesystem, the number of bits used by the maximum file
logical block number is computable from the maximum file size and the
block size.  These values are always present in struct super_block.
Therefore, compute it this way instead of using the value from
fscrypt_operations::get_ino_and_lblk_bits.  Since filesystems always
have to set the super_block fields anyway, this avoids having to provide
this information redundantly via fscrypt_operations.

This change is in preparation for adding support for sub-block data
units.  For that, the value that is needed will become "the maximum file
data unit index".  A hardcoded value won't suffice for that; it will
need to be computed anyway.

Link: https://lore.kernel.org/r/20230925055451.59499-4-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>