fsverity: move sysctl registration out of signature.c
Currently the registration of the fsverity sysctls happens in
signature.c, which couples it to CONFIG_FS_VERITY_BUILTIN_SIGNATURES.
This makes it hard to add new sysctls unrelated to builtin signatures.
Also, some users have started checking whether the directory
/proc/sys/fs/verity exists as a way to tell whether fsverity is
supported. This isn't the intended method; instead, the existence of
/sys/fs/$fstype/features/verity should be checked, or users should just
try to use the fsverity ioctls. Regardless, it should be made to work
as expected without a dependency on CONFIG_FS_VERITY_BUILTIN_SIGNATURES.
Therefore, move the sysctl registration into init.c. With
CONFIG_FS_VERITY_BUILTIN_SIGNATURES, nothing changes. Without it, but
with CONFIG_FS_VERITY, an empty list of sysctls is now registered.
Link: https://lore.kernel.org/r/20230705212743.42180-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>