block/cow.c: fix warnings with _FORTIFY_SOURCE

block/cow.c: fix warnings with _FORTIFY_SOURCE

CC    block/cow.o
cc1: warnings being treated as errors
block/cow.c: In function 'cow_create':
block/cow.c:251: error: ignoring return value of 'write', declared with attribute warn_unused_result
block/cow.c:253: error: ignoring return value of 'ftruncate', declared with attribute warn_unused_result
make: *** [block/cow.o] Error 1

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>