fsverity: simplify handling of errors during initcall

fsverity: simplify handling of errors during initcall

Since CONFIG_FS_VERITY is a bool, not a tristate, fs/verity/ can only be
builtin or absent entirely; it can't be a loadable module.  Therefore,
the error code that gets returned from the fsverity_init() initcall is
never used.  If any part of the initcall does fail, which should never
happen, the kernel will be left in a bad state.

Following the usual convention for builtin code, just panic the kernel
if any of part of the initcall fails.

Link: https://lore.kernel.org/r/20230705212743.42180-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>