scsi: libfc: fix seconds_since_last_reset calculation

The fc_get_host_stats() function contains a complex conversion from
jiffies to timespec to seconds. As we try to get rid of uses of struct
timespec, we can clean this up and replace it with a simpler
computation.

Simply dividing the difference in jiffies by HZ is not only much more
efficient, it also avoids a problem that causes the
seconds_since_last_reset value to be incorrect if jiffies has overrun
since the 'boot_time' value was recorded.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>