Make it clear to Coverity that the tmp buffer in switch_ioscheduler() is \0-terminated

Make it clear to Coverity that the tmp buffer in switch_ioscheduler() is \0-terminated

The 'tmp' buffer used by switch_ioscheduler() will be '\0'-terminated
because the I/O scheduler name read from sysfs has much fewer characters
than the buffer size (256). Make it clear to Coverity that it is
guaranteed that the 'tmp' buffer is '\0'-terminated. Additionally, fix
the order of the fread() arguments. This patch fixes Coverity ID #24132.

Fixes: b44b9e45cf38 ("Clear sysfs path before reading current ioscheduler from sysfs")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>