target: don't call an unmap callback if a range length is zero
If a length of a range is zero, it means there is nothing to unmap and we can skip this range. Here is one more reason, why we have to skip such ranges. An unmap callback calls file_operations->fallocate(), but the man page for the fallocate syscall says that fallocate(fd, mode, offset, let) returns EINVAL, if len is zero. It means that file_operations->fallocate() isn't obligated to handle zero ranges too. Signed-off-by: Andrei Vagin <avagin@openvz.org> Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
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