fsverity: use EFBIG for file too large to enable verity

Currently, there is an implementation limit where files can't have more
than 8 Merkle tree levels.  With SHA-256 and 4K blocks, this limit is
never reached, since a file would need to be larger than 2**64 bytes to
need 9 levels.  However, with SHA-512, 9 levels are needed for files
larger than about 1.15 EB, which is possible on btrfs.  Therefore, this
limit technically became reachable when btrfs added fsverity support.

Meanwhile, support for merkle_tree_block_size < PAGE_SIZE will introduce
another implementation limit on file size, resulting from the use of an
in-memory bitmap to track which Merkle tree blocks have been verified.

In any case, currently FS_IOC_ENABLE_VERITY fails with EINVAL when the
file is too large.  This is undocumented, and also ambiguous since
EINVAL can mean other things too.  Let's change the error code to EFBIG,
which is much clearer, and document it.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20221223203638.41293-5-ebiggers@kernel.org