file-posix: allow -EBUSY errors during write zeros on raw block devices

On Linux, fallocate(fd, FALLOC_FL_PUNCH_HOLE) when it is used on a block device,
without O_DIRECT can return -EBUSY if it races with another write to the same page.

Since this is rare and discard is not a critical operation, ignore this error

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20201111153913.41840-2-mlevitsk@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>