sysfs: upgrade OOB write by buggy .show hook into WARNing

sysfs: upgrade OOB write by buggy .show hook into WARNing

Buggy .show hook will get just 1 line of dmesg:

	fill_read_buffer: ext4_attr_show+0x0/0x600 returned bad count

It may or may not oops later in some unrelated process.

But buggy .show hook most likely is corrupting random memory past sysfs
buffer therefore deserving more. WARN, make it more visible and let
QA machines panic earlier.

Also, delete useless cast -- "count" is >=0 at this point.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/3cc3e8c6-c6e8-4625-a88f-f5708b935dab@p183
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>