netconsole: return count instead of strnlen(buf, count) from store callbacks

netconsole: return count instead of strnlen(buf, count) from store callbacks

Several configfs store callbacks in netconsole end with:

	ret = strnlen(buf, count);

This under-reports the number of bytes consumed when the input
contains an embedded NUL within count, telling the VFS that fewer
bytes were written than userspace actually handed in. A conformant
partial-write loop would then retry the trailing bytes against a
callback that has already accepted them.

Every other configfs driver in the tree returns count directly from
its store callbacks once parsing has succeeded, including
drivers/nvme/target/configfs.c, drivers/gpio/gpio-sim.c,
drivers/most/configfs.c, drivers/block/null_blk/main.c,
drivers/pci/endpoint/pci-ep-cfs.c, and the rest of the configfs
users. netconsole was the outlier (along with
drivers/infiniband/core/cma_configfs.c, which has the same latent
issue).

Align netconsole with the rest of the configfs ecosystem: return
count once the parser/validator has accepted the input. The numeric
and boolean parsers (kstrtobool, kstrtou16, mac_pton,
netpoll_parse_ip_addr) have already validated the meaningful prefix;
any trailing bytes are padding and should simply be reported as
consumed.

Fixes: 0bcc1816188e ("[NET] netconsole: Support dynamic reconfiguration using configfs")
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260427-netconsole_ai_fixes-v2-1-59965f29d9cc@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>