blkdiscard: fix compiler warnings [-Wmaybe-uninitialized]

blkdiscard: fix compiler warnings [-Wmaybe-uninitialized]

sys-utils/blkdiscard.c: In function 'main':
sys-utils/blkdiscard.c:304:33: warning: 'now.tv_usec' may be used uninitialized in this function [-Wmaybe-uninitialized]
sys-utils/blkdiscard.c:152:17: note: 'now.tv_usec' was declared here
sys-utils/blkdiscard.c:305:37: warning: 'now.tv_sec' may be used uninitialized in this function [-Wmaybe-uninitialized]
sys-utils/blkdiscard.c:152:17: note: 'now.tv_sec' was declared here
sys-utils/blkdiscard.c:304:33: warning: 'last.tv_usec' may be used uninitialized in this function [-Wmaybe-uninitialized]
sys-utils/blkdiscard.c:152:22: note: 'last.tv_usec' was declared here
sys-utils/blkdiscard.c:305:65: warning: 'last.tv_sec' may be used uninitialized in this function [-Wmaybe-uninitialized]
sys-utils/blkdiscard.c:152:22: note: 'last.tv_sec' was declared here

Signed-off-by: Karel Zak <kzak@redhat.com>