9p: sysfs_init: don't hardcode error to ENOMEM

v9fs_sysfs_init() always returned -ENOMEM on failure;
return the actual sysfs_create_group() error instead.

Signed-off-by: Randall P. Embry <rpembry@gmail.com>
Message-ID: <20250926-v9fs_misc-v1-3-a8b3907fc04d@codewreck.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>