hwrng: core - use sysfs_emit_at in rng_available_show

hwrng: core - use sysfs_emit_at in rng_available_show

Replace strlcat() with sysfs_emit_at() in rng_available_show() and add
'int len' to keep track of the number of bytes written. sysfs_emit_at()
is preferred for formatting sysfs output because it provides safer
bounds checking.

Inline mutex_lock_interruptible() and drop the now-unused local error
variable. Remove the unnecessary 'buf' NUL initialization. Return 'len'
directly instead of strlen(buf).

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>